spliters

[Rick Kirchner]

Does anyone have a formula to approximate the amount of downforce created by a splitter? I have the McBeath and Katz books and there are no formulas I can find, or even any approximations. Seems like everything out there is done with real-world data.

I have a short diesel pusher RV, and the combination of short wheelbase and the monster Cat hanging off the back can make handling a bit sketchy. The manufacturer installed about 200 lb of steel between the front frame rails. I added a 3x4 tube bumper (full of brick right now but collecting rebar) and a 25 gallon Ag sprayer behind the front cap which is also used to cool off the diesel on long hot climbs.

But let the fuel get below a half-tank on a windy day and it gets tiring.

So given how flat the front is, thought I could make a 9 ft wide splitter. The cap is 1/4" thick, so supporting it shouldn't be too difficult. But there's no reason to do it if I can't get more than a couple hundred pounds at 60mph.

[phantomjock]

Might be easier - and nearly as effective to generate that low pressure you seek under the front with an air dam. Here some material you can order and fasten pretty easily:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

All*Star Performance plastic roll. You may need some support structure behind? No-say.
Would be interesting hearing any quantitative results like mpg change, speed deltas, and butt-dyno observed effectiveness too of course.

Another thought - if it is the rear end wag-the-dog, you could consider Vortex Generators.

Cheers - Jim

[stownsen914]

I've understood that for racecar applications, effectiveness of a splitter is highly dependent on the shape of the car's nose. For an RV application, I'd think it would need to be pretty large to be effective, but possible. It might need to be more of a spoiler than a splitter (possibly splitting hairs on the definition, no pun intended). If there is room for something 6-8" that you could put at a 45 degree forward facing, angled plank across the width of the vehicle, I could see it being a "barn door in the wind."

[Rick Kirchner]

Would be interesting hearing any quantitative results like mpg change, speed deltas, and butt-dyno observed effectiveness too of course.

Another thought - if it is the rear end wag-the-dog, you could consider Vortex Generators.

I was really going for weight on the front end when it is needed the most - at speed. Hitting a bump when in the wind can be unsettling - as is driving next to semis when I don't have the race trailer on. The trailer tends to act like the fletching on an arrow and keep the wag to a minimum.

Usually splitter are used with an air dam, but I'm not sure its always needed. The issue with a dam on a vehicle like this is going over obstacles.

I'm considering airtabs. Takes 70 of them. What I'd really like to do is have a means of linking the airtabs together such that I can get rid of the cap/sidewall joint cover and get a twofer.

[QUOTE]I could see it being a "barn door in the wind."[QUOTE].

It is the veritable definition of a barn door in the wind. And from everything I read, the bigger and blunter the nose the better the splitter should work

[J Leonard]

Check out the European cabover truck tractors for air dams. DAF, Renault and Iveco.

[Rick Kirchner]

They have about the same amount of clearance under the "bumper" as my RV.

[stvsxm]

They have about the same amount of clearance under the "bumper" as my RV.

rick, i may be misunderstanding but it seems to me that you have a significant polar moment of inertia problem. thats a linear horizantal force that ultimately sets up some sort of harmonic with the sidewalls of the tires and tries to kill you. thats not something that you solve with aero. aero is non linear... the faster you go the more you get in a some to sort of exponential curve thats a function of about a million variables you have going on. think of it this way.... forget the splitter and assume you hang a wing under there... now you have a known down load /mph .... thats the good news... the bad news is that your solution will only be correct at a given speed relative to fuel load and be wrong all the rest of the time.. i would think the splitter would be much worse... i have no idea at all what your rig looks like but if it were mine , i would be looking to somehow get the mechanical cg more central ... multiple smaller fuel tanks throughout the chassis some how or something.... i see your problem as a mechanical balance/cg issue that you aren't going to fix with aero...

[DaveW]

rick, i may be misunderstanding but it seems to me that you have a significant polar moment of inertia problem. ...

I agree 100%. It's similar to having the trailer weight distribution too far rearward. You are trying to control a pendulum (or flywheel-like mass distribution) with aero, and as Steve said, and IMO, aero is not likely to cure that to your satisfaction.

The most-likely-to-succeed cure is more distance between the axles, or more of the mass near the center. Probably not what you wanted to hear.

You could also try to find one of the add-on rear (behind the rear axle) air-ride auxiliary axles like what's used on concrete trucks, etc.

That would likely work if you could get a setup that would fit.

[Rick Kirchner]

"Polar moment of inertia problem" - well that's one way to put it I guess. As it's usually described, you have a lot of weight hanging off the back with a considerable sail area, and insufficient load on the front. This leads to what's called "weathervaning" in winds and sidedraft around trucks.

Mechanically, there's no easy fix other than to do what you can to improve weight and balance, because adding wheelbase or axles really isn't in the cards.

in 2010, before any modifications, the axle weights were 7140/16120. Last time I weighed the whole rig in 2020 (trailer, full fuel, water ballast full, water tanks full) it was 6800/18180 and 5660 for the trailer. Total trailer weight is about 6900, so about 1300 tongue weight, and the coach has air bags and automatic leveling, so some of that gets transferred to the front.

With the water and bumper I've added about 400 lb to the front. If I swap out the concrete for rebar in the bumper, I can up that to about 500 lb. I've considered stuff like adding a couple of batteries to the bumper which would also help with boondocking, but you do have to get under the cap periodically to maintain the AC and other items, and adding more stuff at this point just makes it disassembly nightmare of heavy stuff.

I've considered moving the genset to under the cap, but that entails not only an enormous amount of fabrication and a bunch of challenging wiring runs, but also the removal of the existing 250lb of ballast welded between the frame rails (roughly 6x6x24 inches of solid bar!). So a loss of about 400 lb from the rear and a net gain of 150 on the front, while inducing a lot of fabrication and maintenance headaches. In the very last version of this coach the manufacturer did exactly that, complete with a hydraulic extender mechanism and a door in the front cap.

But aero has to be a major part of the solution, because at 7140/16120 the thing was damn near undriveable. A steering stabilizer and attention to the alignment helped, but it's much easier to drive with the tow car on and even better with the trailer despite having an even worse weight distribution.

The trailer is a massive drag load. I ordered it with an extended tongue, which makes it so you don't have to worry about jacking the trailer into the coach when you have the two at 90 degrees, but moves the trailer out of the wind shadow of the RV.

Obviously this thing is a compromise. The purpose behind a small coach with a big engine was to preserve as much HP for towing. Second purpose was that we like to stay and visit state and national parks, and big coaches really aren't compatible with that. At 33' OAL, it's actually has a smaller footprint than a lot of Class C's. I'd had a front-engine gas coach at work and that damn thing was underpowered (Ford 460) and you needed a headset to talk to the person next to you. Not an enjoyable experience.

[DaveW]

So, back to basics.

So your main issue when you are driving the RV by itself? You seem to indicate that by saying it's better with the trailer attached.

In one sentence you say adding weight to the front seems to help, but then you say it's a "weathervaning" issue. Obviously it's a combination of weight distribution and aero.

So, as you say, downforce/weight in the front would help by giving the front tires more traction, but that does not eliminate the rearward center of pressure issue, which causes the weathervaning.

What is the wheelbase? You say the RV is 33 ft long. Knowing the wheelbase would give more substance to the overall issue.

I assume it has dual wheels in the back.

My personal experience with this sort of issue is limited, but on my '74 Chevy G30 van/trailer combo, going from the stock rear wheel width (6.75", IIRC) to 8" wide eliminated a bad sway issue we had passing semis. So maybe there's something you can do laterally stiffening the rear wheels/tires that would have a similar effect.

[BeerBudgetRacing]

in 2010, before any modifications, the axle weights were 7140/16120. Last time I weighed the whole rig in 2020 (trailer, full fuel, water ballast full, water tanks full) it was 6800/18180 and 5660 for the trailer. Total trailer weight is about 6900, so about 1300 tongue weight, and the coach has air bags and automatic leveling, so some of that gets transferred to the front.

Are you running a weight distributing hitch? I don't believe air bags and leveling will transfer weight to the front. I tried that with my truck-trailer. Using a weight distributing hitch and NO airbags actually loaded the front.

[DaveW]

Are you running a weight distributing hitch? I don't believe air bags and leveling will transfer weight to the front. I tried that with my truck-trailer. Using a weight distributing hitch and NO airbags actually loaded the front.

Correct - to transfer weight to the front, one needs what you said. But weight distributing takes load off the rear and moves it to the front, which may not be the right way for rear lateral stiffness, although it is great for rear tire durability.

[stvsxm]

Are you running a weight distributing hitch? I don't believe air bags and leveling will transfer weight to the front. I tried that with my truck-trailer. Using a weight distributing hitch and NO airbags actually loaded the front.

you took the keystrokes right off my keyboard. the load leveling hitches work great at mitigating exactly this issue BUT the numbers he is talking about i would think are beyond even that solution. and i serious question the weather vane analysis. certainly wind isn't helping anything but the massive mechanical imbalance , in my opinion is the overwhelming factor. thats why 5th wheels work so well.... they move all the tongue weight near the cg of the truck and load the front and rear as evenly as possible. the cg of this rig is so far back that rear ward later forces simply overwhelm the ability of the front to cope. a load leveling hitch will certainly help if they manufacture one for aircraft carriers...

[Rick Kirchner]

You cannot use a weight distributing hitch with a load-leveling air bag suspension. Everything you do on the hitch the coach will counter. There are ride-height valves in the system. You put a couple thousand pounds on the rear and it drops and the front comes up - then the back comes back up and the front drops by putting in/taking out air, effectively changing the spring rate.

Wheelbase is 178" (14' 9.5") with an OAL of 33 ft.

To Dave's question - I'd like to improve handling both empty and towing a car. When going long distances it's more convenient to NOT drag a tow car and just rent at the other end (if rental prices ever become reasonable again). You have to constantly watch the tow car for dolly issues, etc, and it takes a couple of MPG off. It's just another thing to get flat tires.

Really don't have what I would construe as a sidewall flex issue - but if I were trying to address that I might consider Super Singles (with moon caps and smoothie fronts that would look badass) but that's a sizable investment.

Would be interesting to look at the CP vs CG issue analytically - with empty weights and the position of the various tanks/weighting options one could do a method of moments calculation. vehicle as a box for CFD? Actually Art could do something like that, I only occasionally impersonate an ME, sometime with dubious results.

[DaveW]

...My personal experience with this sort of issue is limited, but on my '74 Chevy G30 van/trailer combo, going from the stock rear wheel width (6.5", IIRC) to 8" wide eliminated a bad sway issue we had passing semis. So maybe there's something you can do laterally stiffening the rear wheels/tires that would have a similar effect.

I also meant to say that the tires stayed the same. With the wider rims, the sidewall flex decreased dramatically.

[DaveW]

You cannot use a weight distributing hitch with a load-leveling air bag suspension. Everything you do on the hitch the coach will counter. There are ride-height valves in the system. You put a couple thousand pounds on the rear and it drops and the front comes up - then the back comes back up and the front drops by putting in/taking out air, effectively changing the spring rate...

Actually, you CAN use a weight distributing hitch with air suspension, and change the tow vehicle's F-R loading ratio.

As you said, the air suspension is controlled by height control valves.

So say the tow vehicle is at its controlled F&R ride heights w/o the weight distributing hitch tensioned. You then put tension on the weight distributing spring arms. This causes a torque at the hitch, raising the rear of the tow vehicle, pushing the front of it down, and loading the trailer tires more. In response, the height control valves let air out of the rear and put more air into the front, bringing the tow vehicle back to its design ride heights. What is now different is that the rear air springs are at a lower pressure, spring rate, and load, while the fronts are at a higher air pressure, spring rate, and load.

That is what you say you are trying to achieve with the splitter!

[Rick Kirchner]

Okay, but my owners manual says "don't".

Have no idea why....

[Chris Livengood]

Look up those load balancing trailer attachments that add a set of wheels at the hitch. There is no way that wouldn't help keep the nose down a bit. I am not sure if you can use that without a trailer, but look into it.

For the record, my father and I (and my friend Vaughn) were in an accident with our Class A motorhome that was caused by what you are describing. The thing just wouldn't turn or stop because front end stability was zero. So it is something I am more than familiar with. The results were not pretty, but we all moved on with only minor injuries. I'll never own a non-bus based motorhome as a result, that crash was scary. The worst I have ever been privy too, which is saying something after 27 years of driving various racing vehicles. Had my father worn his seat belt (something he religiously does but happened not to do in this instance) he would have been beheaded in front of me and my friend. The seatbelt attachments moved rearward relative to the seat at least a foot in the crash. The crash was at less than 20mph.

Take your safety seriously, that is what I am saying.

[Rick Kirchner]

Yeah, the trailer toads need a trailer.

What brand/type/length of motorhome? There are some Ford chassis that have some horrible handling problems, but they are front-engined. Mostly related to sway bars and I believe the rear trying to steer the front. There's like a 30 page thread on IRV2 that's been going on 10 years about it.

Luckily, our seat belts are embedded in the seats. The fact that there's no structure behind the cap was also a motivating factor to put a bumper under there. The frame on my coach is truly massive, but the house is just a bunch of wood, aluminum, and fiberglass panels.

Some DPs are prone to porpoising. Luckily I've never really had that problem, although sometimes the spring rate in the pedal and throttle response can work together and really start the coach lurching.

[Chris Livengood]

It was a Winnebago, I forget the model. 32' as I recall. This happened nearly 20 years ago, so forgive me for my lack of details.

The large rear overhang much like yours made it handle like dog ****. A little bit of control over the chassis as far as the seesaw effect would have helped a lot. When we crashed we had a trailer full of racing stuff, the trailer (home built, four wheel brakes) and contents all survived without issue. We still tow that trailer with our DSR to vintage races. The motorhome basically exploded. Not in a bomb type of way but rather an inertia type of way. My seat belt was attached as you described, to the seat itself. The drivers' seat had belts attached to the body. Not cool. My father is alive and well and still ****ing **** up, if you met him you'd understand what I mean. It's a compliment.

I will never forget picking up my wrist watch off the asphalt along with a box of cereal. A very odd situation that one. Try to balance that beast of yours out. You posted here because you are concerned. I urge you to continue down that path.

Trailer and DSR to prove I am not a dog on the internet;

[stvsxm]

It was a Winnebago, I forget the model. 32' as I recall. This happened nearly 20 years ago, so forgive me for my lack of details.

The large rear overhang much like yours made it handle like dog ****. A little bit of control over the chassis as far as the seesaw effect would have helped a lot. When we crashed we had a trailer full of racing stuff, the trailer (home built, four wheel brakes) and contents all survived without issue. We still tow that trailer with our DSR to vintage races. The motorhome basically exploded. Not in a bomb type of way but rather an inertia type of way. My seat belt was attached as you described, to the seat itself. The drivers' seat had belts attached to the body. Not cool. My father is alive and well and still ****ing **** up, if you met him you'd understand what I mean. It's a compliment.

I will never forget picking up my wrist watch off the asphalt along with a box of cereal. A very odd situation that one. Try to balance that beast of yours out. You posted here because you are concerned. I urge you to continue down that path.

Trailer and DSR to prove I am not a dog on the internet;

the irony of all this is exactly what you describe.... in the 30 years i spent doing this i was never in much danger of being killed on the race track but considering some of the tow rigs i and a LOT of others were using we often were surprised to get to the race track alive at all... buddy norton and i left providence for road atlanta in some giant dodge sedan i had bought for 300$ with a 55 gallon drum of fuel in the trunk and two spare motors and the tools where the back seat used to be and an open trailer. ... the headlights pointed straight up and the handling was , well... less than precise... but that was when we were young and stupid. im not sure in modern times with what is available for equipment and knowledge there is any reason to be towing with something thats trying to kill you...

[DaveW]

Okay, but my owners manual says "don't".

Have no idea why....

Why? The possibility of making the air system malfunction:

If the load distributing hitch was tensioned to an extreme, it's possible (not likely) that with a light tow vehicle, you might cause a situation where the air suspension would be unable to control the ride heights and the air system would, indeed, be fighting the load distributing hitch, leading to serious issues. So, as with almost anything, one has to use some common sense in this L-D hitch application.

[DaveW]

Yeah, the trailer toads need a trailer.

What brand/type/length of motorhome? There are some Ford chassis that have some horrible handling problems, but they are front-engined. Mostly related to sway bars and I believe the rear trying to steer the front. There's like a 30 page thread on IRV2 that's been going on 10 years about it.

Luckily, our seat belts are embedded in the seats. The fact that there's no structure behind the cap was also a motivating factor to put a bumper under there. The frame on my coach is truly massive, but the house is just a bunch of wood, aluminum, and fiberglass panels.

Some DPs are prone to porpoising. Luckily I've never really had that problem, although sometimes the spring rate in the pedal and throttle response can work together and really start the coach lurching.

Looking at the picture you posted of your rig, there certainly is room (lengthwise, at least) for installation of an air-deployed 3rd axle like I mentioned before to stabilize the motorhome. The caveat is maybe not enough frame strength back there to attach it properly and safely. The vertical space might dictate relatively small diameter tires/wheels, but in your case, anything like this would help a HUGE amount because of the increase of effective wheel base, plus the load redistribution. You'd also have to calculate and set up the 3rd axle for the proper amount of load (maybe manually - independently - inflated) to make it work correctly.

If you could install a setup like this, you could likely take almost all of that ballast off the front, winding up at overall lower rig weight than what you have now.

IMO, this is the only solution that directly addresses both of the issues of too much rearward mass and too short wheelbase while driving either w/ or w/o trailer.

[Ed Womer]

when you started this thread I noticed it and wondered why you might be having this issue. I wondered what your tongue weight might be and if you where raising up the nose with excessive tongue weight. I had an old 89 Winnebago motor home that lets say the frame was not adequate for towing anything other flat towing a light car.

After returning from a trip to New Hampshire taking a second vee along I noticed the basement doors wouldn't stay closed. After getting home I looked the motor home over good and realized it was actually bending. I borrowed some scales from a FC friend and my tongue weight was only 600lbs which is not a lot.

I then looked into a Trailer Toad that is a set of wheels that carry the tongue weight. I shortened the one basement compartment and added a frame from the one they cut off to have a longer compartment, and connected it to the short one that the hitch was attached to. I did by the Trailer Toad and wow what a difference.

Check your tongue weight and lower it by better balance the load in the trailer. This reduces the tail wagging the dog, which might be part of your problem. I still have my old trailer toad and would like to sell it but I am in PA so that would cost you a fortune for shipping.

Since returning to racing, the wife and I decided quickly that motel-ling it wasn't our thing and we found an extremely lightly used 25' Thor motor home and are very happy with it so far. I am using an open trailer for my formula First car but with a balanced load and everything has been fine so far.
good Luck!

Ed

[Rick Kirchner]

That area behind the rear wheels - electrical bay, then the genset on driver's side. Passenger side has the battery box and then the engine maintenance/hydraulics bay with pump, filters, external monitors and jack controls, etc.

One of the considerations of a short coach is lack of basement storage. Get a 40 footer and there's 7 feet more on each side. But then there's no handling issues. From a packaging perspective - a lot like a formula car!

It would be possible, but to do so by moving the genset to the nose, batteries to the driver's side, combine electrical and engine bays as far back as possible and then there would be space for a tag.

I'm thinking I'll start with more front weight via rebar and airtabs on the back - they don't do much for gas milage but everybody that has them says they do a lot for sidedraft off trucks. That's the easy button. If that doesn't do enough, then I'll look at moving the genset into the nose, along with the house batteries. At that point I might be at the front axle limit.

[DaveW]

Rick,

Lots of varying opinions here - good luck getting it resolved.

I looked at the air-tab option for my trailer. I decided not to use them because they would make washing the trailer a PITA, plus having anything sticking out seems to be hazardous to me - I keep running into them. Next thing I know I'm leaking blood all over stuff.

I also took a cue from over-the-road semi's. I see no air-tabs on those, but quite a few "trailer-tails." That said to me the air-tabs didn't do much regarding trailer aero drag. Your situation is different, so if you use them I hope it's worth the PITA they are likely to be.

[Mark_Silverberg]

Rick,



I looked at the air-tab option for my trailer. I decided not to use them because they would make washing the trailer a PITA, plus having anything sticking out seems to be hazardous to me - I keep running into them. Next thing I know I'm leaking blood all over stuff.

I also took a cue from over-the-road semi's. I see no air-tabs on those, but quite a few "trailer-tails." That said to me the air-tabs didn't do much regarding trailer aero drag.

My experience with Airtabs is that they are very good and making the trailer less sensitive to turbulence - if in far right lane or far left lane - but if trucks on both sides there is not much difference than having no tabs. I do see trucks with airtabs but much more with trailer tails - of course most of the tails I see are not deployed. I have no issues washing the trailer with the tabs and the do not stick out far at all and are not sharp in any way.

I find that rear overhang is one of the biggest influences on stability when towing. My Nissan NV2500 V8 High roof van was a great tow vehicle - largely because of its very short rear wheel centerline to hitch ball measurement.

[problemchild]

Driving a garden shed on wheels down a highway is an extreme sport! The passenger seat in my old RV had the seatbelt attached to the seat, but the seat had rusted off the floor. It was funny as no one ever got hurt. We called it the "WES" (Wife ejection seat). "Honey, make sure you have your seatbelt on."

[stvsxm]

Rick,

Lots of varying opinions here - good luck getting it resolved.

I looked at the air-tab option for my trailer. I decided not to use them because they would make washing the trailer a PITA, plus having anything sticking out seems to be hazardous to me - I keep running into them. Next thing I know I'm leaking blood all over stuff.

I also took a cue from over-the-road semi's. I see no air-tabs on those, but quite a few "trailer-tails." That said to me the air-tabs didn't do much regarding trailer aero drag. Your situation is different, so if you use them I hope it's worth the PITA they are likely to be.

thats funny. i looked up the air tabs and had to laugh. that is exactly what the pratt guys were working on. theirs were considerably more complex but the phenomena is called " interlaced trailing edge vortices" . properly designed they create vortices that weave themselves together and make a shape think is long and streamlined.. the data they showed me from wind tunnel work indicated that they had made a semi trailer have the same CD as a pontiac firebird of the same era. theirs weren't typical vortex generators that you see with the air tabs but scallop shapes with very complex geometries "scooped" into the trailing edges of the " box. theirs were all about drag reduction... not downforce. i think the idea of weight distribution as a solution is the right approach. i think the idea of just adding anvils to the front bumper sounds more like a " i'm stuck on a desert island, what do i do ? " sort of solution.... good to get you home from somewhere once but a really bad idea in the long run... " engineering rule number 1... solve the problem, not the symptoms..."

addendum... i just looked up the trailer toad thing and those guys seem to be the category experts on this problem . if i were you, rick, i would get them on the phone and ask them the same questions you asked here. it seems like that for 4k you can write a check and live happilly ever after. thats engineering rule number two... " any problem you can solve with money isn't a problem at all "

[DaveW]

... " engineering rule number 1... solve the problem, not the symptoms..."

addendum... I just looked up the trailer toad thing and those guys seem to be the category experts on this problem . if i were you, rick, I would get them on the phone and ask them the same questions you asked here. it seems like that for 4k you can write a check and live happily ever after. that's engineering rule number two... " any problem you can solve with money isn't a problem at all "

Rick's motorhome has a stability issue that, IIRC, he says seems worse w/o the trailer attached, so he needs a solution for that w/o the "toad."

[Rick Kirchner]

Couple of things about Airtabs. One of the reasons you don't see them on semis is because most of the guys are usually hauling other people's trailers, and the trailer panels/boattail are more effective at drag reduction. Everything I've read about airtabs (despite the manufacturer's claims) are that they are very effective at sway reduction, but the drag reduction is almost unnoticeable.

You'll see more Airtabs on trucks that haul their own trailers.

One of the reasons you see so many trailer tails is that CA requires drag reduction devices on any box semi trailer in the state - along with the big underside panels. Most fleets don't want to keep track of some trailers that can go and others that can't, so they put the devices on everything. Besides, they actually do work.

I'm not having stability issues when I'm towing the race trailer despite the fact that it makes the weight imbalance worse. The drag associated with the trailer must move the CP much farther back. It's when the coach is towing nothing or just our Honda on a dolly - then crosswinds and semis can make it a tiring day.

You would not have believed how bad it was before I added the steering stabilizer (a shock absorber with two centering springs that's over 2 feet long and 3" in dia) paid close attention to the alignment and added the weight up front! Not to mention the inability to stop. When they built the thing the brake pedal linkage was adjusted such that it would immediately go over center at the MC. Thankfully I figured that out in a hurry with a little help from Carroll Smith.

Ed, I bet your Winnie was a front engine coach. There's frequently only one wheelbase from each manufacturer of those, so length gets added behind the rear bumper with frame extensions - same with most class C's. you can literally tear the back of the house off if the hitch is overloaded or rust gets in those extensions - which are frequently pretty sketchy to start with. A 40' front-engine class A has crazy rear overhang, but it's all hollow-core wood and such, not much weight. The trailer toad was designed exactly for the situation where the trailer has more tongue weight than the coach can carry.

[stvsxm]

Couple of things about Airtabs. One of the reasons you don't see them on semis is because most of the guys are usually hauling other people's trailers, and the trailer panels/boattail are more effective at drag reduction. Everything I've read about airtabs (despite the manufacturer's claims) are that they are very effective at sway reduction, but the drag reduction is almost unnoticeable.

You'll see more Airtabs on trucks that haul their own trailers.

One of the reasons you see so many trailer tails is that CA requires drag reduction devices on any box semi trailer in the state - along with the big underside panels. Most fleets don't want to keep track of some trailers that can go and others that can't, so they put the devices on everything. Besides, they actually do work.

I'm not having stability issues when I'm towing the race trailer despite the fact that it makes the weight imbalance worse. The drag associated with the trailer must move the CP much farther back. It's when the coach is towing nothing or just our Honda on a dolly - then crosswinds and semis can make it a tiring day.

You would not have believed how bad it was before I added the steering stabilizer (a shock absorber with two centering springs that's over 2 feet long and 3" in dia) paid close attention to the alignment and added the weight up front! Not to mention the inability to stop. When they built the thing the brake pedal linkage was adjusted such that it would immediately go over center at the MC. Thankfully I figured that out in a hurry with a little help from Carroll Smith.

Ed, I bet your Winnie was a front engine coach. There's frequently only one wheelbase from each manufacturer of those, so length gets added behind the rear bumper with frame extensions - same with most class C's. you can literally tear the back of the house off if the hitch is overloaded or rust gets in those extensions - which are frequently pretty sketchy to start with. A 40' front-engine class A has crazy rear overhang, but it's all hollow-core wood and such, not much weight. The trailer toad was designed exactly for the situation where the trailer has more tongue weight than the coach can carry.

im sorry... i clearly misunderstood. i thought this whole thing was trailer/package related... but to your point... if its just the coach , you can't be the only one with this issue... everybody with one of those must have the problem. what do they do ?

[Mike B]

Couple of things about Airtabs. One of the reasons you don't see them on semis is because most of the guys are usually hauling other people's trailers, and the trailer panels/boattail are more effective at drag reduction. Everything I've read about airtabs (despite the manufacturer's claims) are that they are very effective at sway reduction, but the drag reduction is almost unnoticeable.

You'll see more Airtabs on trucks that haul their own trailers.

That's part of the story. In many Truckload fleets there is a roughly 1:4 ratio of tractors to trailers which means at any given time only 1/4 of the trailers are in use. Therefore, any MPG benefit of an aerodynamic aid is cut by 75%, or rather the cost of the aid is quadrupled. The benefit needs to be significant to justify the investment. Trailer skirts work well so you see them a lot more. Boat tails also work but are a PITA to deploy so they are usually folded out of the way.
In a past life we did SAE Type II fuel testing on air tabs on class 8 trucks and the result was a 0% improvement. I don't recall if the test drivers mentioned anything about sway but if it's too windy the tests are invalid anyway. Sway isn't really a concern on an 18 wheeler.

[Rick Kirchner]

im sorry... i clearly misunderstood. i thought this whole thing was trailer/package related... but to your point... if its just the coach , you can't be the only one with this issue... everybody with one of those must have the problem. what do they do ?

Your average RV owner out there is pretty clueless. Not a lot of engineers (at least of the kind that will dig into the problem). I posted my process to fix the brake issue and you wouldn't have believed the number of nervous nellies that were sure I'd get sued over it. This is the same community that still debates whether or not its a good idea to park your coach on concrete without something under the tires to prevent "leaching". i think leeches are probably part of their medical regimen too. Its truly a cesspool of dumbassery.

There's just the usual word of mouth stuff "I heard short coaches don't drive very well".....

As an owner, that's hard to assess until you've laid your money down. Most test drives aren't going to show the problem. Now that I have 10 years of driving one under my belt I'm better equipped to assess it, but when you first start out, it's all white knuckle. Takes about 20-40 hrs before you get truly comfortable.

The problem increases with speed - the coach is pretty comfortable on its own with no side wind at 65. 65 in a herd of trucks is disconcerting. 75 going downhill is white knuckle. So my thought was if it is speed related an aero solution might be appropriate.

[stvsxm]

Your average RV owner out there is pretty clueless. Not a lot of engineers (at least of the kind that will dig into the problem). I posted my process to fix the brake issue and you wouldn't have believed the number of nervous nellies that were sure I'd get sued over it. This is the same community that still debates whether or not its a good idea to park your coach on concrete without something under the tires to prevent "leaching". i think leeches are probably part of their medical regimen too. Its truly a cesspool of dumbassery.

There's just the usual word of mouth stuff "I heard short coaches don't drive very well".....

As an owner, that's hard to assess until you've laid your money down. Most test drives aren't going to show the problem. Now that I have 10 years of driving one under my belt I'm better equipped to assess it, but when you first start out, it's all white knuckle. Takes about 20-40 hrs before you get truly comfortable.

The problem increases with speed - the coach is pretty comfortable on its own with no side wind at 65. 65 in a herd of trucks is disconcerting. 75 going downhill is white knuckle. So my thought was if it is speed related an aero solution might be appropriate.

fair enough.... so let me ask you this by way of data gathering.... what exactly do you think is happening ?... in a purely mechanical sense ? i mean.... when the sway or instability starts , whats really going on ? its a big mass... high cg and weird weight distribution.... all the external forces act on those levers... so whats moving ? do you think the sidewalls of the tires are deflecting ? is it just a rotation around the long axis where the suspension on one side collapses and the other droops ? i guess im saying " look, this is a mechanical system like any other . it has links and levers all affected by the external forces like any other.... so the question becomes , " what is actually moving and how do you control it or damp it ?" if its side walls of the tires , try running a ton of air pressure as a test. if its undamped suspension, try to find shocks that have a ton of rebound control. you are surrounded by some of the smartest and best " out of the box " thinkers here. why not just look at this as simple free body diagram whose forces and vectors need simple mechanical solutions ? lets say this was your race car and it was all mushy and lazy at turn in... what would you do ? you would put compression damping in the front shocks and some re-bound in the rear shocks and stiffen the thing up some as the big first step because your brain would intuitive visualize what the chassis wanted for control influences. or would you simply crank more wing into it ? why is your motorhome any different ? just bigger forces in different places... the solution starts by understanding whats really happening. i still don't think you could bring sufficient aero forces to bear to make the difference but thats just an intuitive feeling and i could be completely wrong and you could be completely right.

[Rick Kirchner]

fair enough.... so let me ask you this by way of data gathering.... what exactly do you think is happening ?... in a purely mechanical sense ? ....... " what is actually moving and how do you control it or damp it ?"

Those are some great questions.

Given that I only have two tools to work with - throttle and steering wheel - when it happens I usually get off the throttle and steer a bunch. There's a LOT of steering that goes on with this coach - it's pretty numb. There's some caster added to the right side to deal with road crown, and it's probably not enough but I don't know what the practical limit is to that given the size of the front beam. This is a recirculating ball system with a drag link - pretty much what you see on a school bus.

Whether getting off the throttle is an attempt in my brain to get weight back on the front or just a commons-sense reaction I can't say.

The shock question is an interesting one. I has Bilsteins, about 80K mi on them. A lot of the community reports "better ride" with a product Koni sells with VFD. Given that there's not a lot of customer demand for custom valved motorhome shocks and likely nobody to know what to valve them to, you do have me thinking about scouring the used off-road market out here to see what I might scare up.

From a dynamics perspective, when a truck first approaches you can feel his pressure wave before he truck gets along side. the nose of the coach starts to steer into the left lane as the pressure pushes on the back corner and all that sail area behind the rear axle. I think it's trying to rotate about the rear axle. Besides the push from the bow wave I could see the coach rolling to the right, so now I'l need to pay more attention to that.

Once the semi comes alongside about the rear axle things calm down and then the coach starts to push toward the shoulder. Some configurations of trucks are worse than others - car haulers give me a real beating.

When pulling the trailer you get this twice with a phase reversal - trailer nose is first pushed to the left, which pushes the coach's nose to the right and then reverses when the truck reaches the back corner of he coach, although it's not as pronounced. There's not as much effect when towing a car.

I'd love to have data acquisition for this! Four shock pots, a steering sensor, and a camera would sure tell a lot! or even five cameras.

[stvsxm]

Those are some great questions.

Given that I only have two tools to work with - throttle and steering wheel - when it happens I usually get off the throttle and steer a bunch. There's a LOT of steering that goes on with this coach - it's pretty numb. There's some caster added to the right side to deal with road crown, and it's probably not enough but I don't know what the practical limit is to that given the size of the front beam. This is a recirculating ball system with a drag link - pretty much what you see on a school bus.

Whether getting off the throttle is an attempt in my brain to get weight back on the front or just a commons-sense reaction I can't say.

The shock question is an interesting one. I has Bilsteins, about 80K mi on them. A lot of the community reports "better ride" with a product Koni sells with VFD. Given that there's not a lot of customer demand for custom valved motorhome shocks and likely nobody to know what to valve them to, you do have me thinking about scouring the used off-road market out here to see what I might scare up.

From a dynamics perspective, when a truck first approaches you can feel his pressure wave before he truck gets along side. the nose of the coach starts to steer into the left lane as the pressure pushes on the back corner and all that sail area behind the rear axle. I think it's trying to rotate about the rear axle. Besides the push from the bow wave I could see the coach rolling to the right, so now I'l need to pay more attention to that.

Once the semi comes alongside about the rear axle things calm down and then the coach starts to push toward the shoulder. Some configurations of trucks are worse than others - car haulers give me a real beating.

When pulling the trailer you get this twice with a phase reversal - trailer nose is first pushed to the left, which pushes the coach's nose to the right and then reverses when the truck reaches the back corner of he coach, although it's not as pronounced. There's not as much effect when towing a car.

I'd love to have data acquisition for this! Four shock pots, a steering sensor, and a camera would sure tell a lot! or even five cameras.

how about a rear sway bar the size of your leg ? i have seen two devices that solve this sort of issue... in office buildings the use giant mass dampers and in the marine industry the use gyro stabilizers which are sort of like an active sway bar... the last one i actually saw was on a russian mega yacht down here and cost 400k but that boat was rock steady in 4 foot swells.... but either way... i think the off road shock market is a good idea.... maybe even multiples like they do on the baha trucks...

heres a thought that just occurred to me.... how about putting some rake in it... drop the nose as much as humanly and practically possible... you get your cp shift and a lot more positive nose...

[Charles Warner]

Assume trading up is not an option?

[DaveW]

Rick,

IMO, you are dealing more with low-frequency, almost static phenomena. They happen over periods of multiple seconds, not in fractions of seconds like in a race car.

So my thoughts trend towards stuff that changes the ability to withstand relatively large, but slow, yaw movements and less toward things that react to velocity, like shocks. That's why I suggested the 3rd axle that would resist the rear of the coach getting moved by air pressure, which seems to me to be the predominant issue. That also would load the front axle more and make steering response better.

Adding load to the front axle, as you said, also helps by increasing the available force needed to counteract the yaw.

All of the other things that have been mentioned are, IMO, much less likely to have enough of a positive effect to make the coach comfortable to drive in side-pressure situations.

[Rick Kirchner]

Assume trading up is not an option?

Not given the current market, and the short coach does have its other advantages.